The subject application claims benefit of the earlier filing dates of Japanese Patent Application Nos.Hei 11-74258 and 2000-32359 filed on Feb. 14, 1999 and Feb. 9, 2000 under the Paris Convention, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to a switching device provided with a function for detecting a break in a load of lights.
2. Description of the Related Art
FIG. 1 shows a switching device according to a prior art used for a load such as a lamp, having a function for detecting a break in the load. The switching device has an element (an FET in this example) QF for controlling the supply of power from a power source such as a battery of an automobile to a load such as a lamp, The power source 101 provides an output voltage VB. The power source 101 is connected to an end of a shunt resistor RS. The other end of the shunt resistor RS is connected to a drain D of the element QF. A source S of the element QF is connected to the load 102. A driver 901 detects a current flowing through the shunt resistor RS and drives the element QF accordingly. An A/D converter 902 and a microcomputer (CPU) 903 receive a curre.nt value detected by the driver 901, and according to the current value, determine whether or not an overcurrent is flowing to the element QF and whether or not the load 102 involves a break. The element QF may have a temperature sensor to achieve an overheat breaking function.
A zener diode ZD1 keeps a voltage of 12 V between the gate G and source S of the element QF, to bypass an overvoltage so that the overvoltage may not be applied to the gate of the element QF. The driver 901 includes differential amplifiers 911 and 913 serving as a current monitor circuit, a differential amplifier 912 serving as a current limiter, a charge pump 915, and a driver 914. The driver 914 receives an ON/OFF control signal from the microcomputer 903 and an overcurrent signal from the current limiter and drives the gate of the element QF through an internal resistor RG (not shown). A voltage drop occurring at the shunt resistor RS is detected by the microcomputer 903 through the differential amplifiers 911 and 913 and A/D converter 902. If the voltage drop is above a normal level corresponding to a normal current value, the microcomputer 903 determines an overcurrent, and if it is below the normal level, the microcomputer 903 determines a break in the load 102.
This prior art must have the shunt resistor RS connected in series to a power line, to detent a current in the power line. The shunt resistor RS causes a large heat loss if a current passing through the power line is large.
The shunt resistor RS, A/D converter 902, microcomputer 903, etc., that are imperative for the prior art require a large space and are expensive, thereby increasing the size and cost of the switching device.
An object of the present invention is to provide a semiconductor switching device that is easy to integrate, inexpensive, and capable of detecting a current in a power line without a shunt resistor connected to the power line, minimizing a heat loss, and detecting a break in a load such as a lamp.
In order to accomplish the objects, an aspect of the present invention provides a semiconductor switching device having a first semiconductor element, a second semiconductor element, and a comparator. The first semiconductor element has a first main electrode, a second main electrode, and a control electrode. This second main electrode is connected to lamps serving as a load. The second semiconductor element has a first main electrode connected to the first main electrode of the first semiconductor element, a control electrode connected to the control electrode of the first semiconductor element, and a second main electrode connected to a circuit that is composed of a resistor and a constant current source that are connected in parallel with each other. The comparator compares potentials of the second main electrodes of the first and second semiconductor elements with each other. If the potential of the second main electrode of the first semiconductor element is higher than the potential of the second main electrode of the second semiconductor element, it is determined that there is a break in the lamps. The first and second semiconductor elements may be FETS, SITs (static induction transistors), or BJTs (bipolar junction transistors). Alternatively, the first and second semiconductor elements may be MOS composite elements such as ESTs (emitter switched thyristors) and MCTs (MOS controlled thyristors), or insulated gate power elements such as IGBTs. These elements may be any one of n- and p-channel types. The first main electrode is one of the emitter and collector electrodes of a BJT or an IGBT, or one of the source and drain electrodes of an IGFET such as a MOSFET and MOSSIT. The second main electrode is the other of the emitter and collector electrodes of the BJT or IGBT, or the other of the source and drain electrodes of the IGFET. If the first main electrode is an emitter electrode, the second main electrode is a collector electrode. If the first main electrode is a source electrode. the second main electrode is a drain electrode. The control electrode is a base electrode of the BJT, or a gate electrode of the IGBT or IGFET.
According to the aspect, a resistance value of the resistor and a current value of the constant current source are set such that the potential difference between the second main electrodes of the first and second semiconductor elements will be zero if a current flowing through the first semiconductor element has an intermediate value between a normal current value and a break current value.
A break in the lamps is detected according to the value of a current passing through the lamps. The intermediate current value between a normal current value and a break current value is IDAS, which is used as a break reference value. If the first semiconductor element is a power MOSFET, a terminal voltage (drain-source voltage) of the power MOSFET is expressed as xe2x80x9cRonAxc3x97IDAxe2x80x9d where RonA is, the ON resistance of the power MOSFET and IDA is a drain current thereof. Similarly, a terminal voltage of the second semiconductor element is expressed as xe2x80x9cRonBxc3x97IDBxe2x80x9d where RonB is the ON resistance of the second semiconductor element and IDB is a drain current thereof. The drains of the first and second semiconductor elements are connected to each other, and the gates thereof are also connected to each other. When the break reference current IDAS is flowing to the first semiconductor element, the current value IDB of the second semiconductor element is so set as to zero the potential difference between the sources (the second main electrodes) of the first and second semiconductor elements, and the following is established:
RonBxc3x97IDB=RonAxc3x97IDAxe2x80x83xe2x80x83(1)
If the lamps have no break, the following is established:
RonAxc3x97IDA greater than RonBxc3x97IDBxe2x80x83xe2x80x83(2)
If the lamps have a break, the following is established:
RonAxc3x97IDA less than RonBxc3x97IDBxe2x80x83xe2x80x83(3)
Namely, the potential difference between the sources (the second main electrodes) of the first and second semiconductor elements is indicative of a break in the lamps.
The drain current IDB is expressed as follows:
IDB=(RonA/RonB)IDASxe2x80x83xe2x80x83(4)
The break reference current IDAS changes according to a power source voltage. As the power source voltage increases, the break reference current IDAS increases. However, as the source voltage increases, the temperature of lamp filaments increases to increase the resistance of the lamp filaments, and therefore, the break reference current IDAS is not simply proportional to the. power source voltage. To realize IDB that satisfies the expression (4), the circuit composed of the resistor and constant current source that are connected in parallel with each other is inserted between the source of the second semiconductor element and the ground.
According to the aspect, the first and second semiconductor elements, comparator, and constant current source may be integrated on a semiconductor chip. The driver may also be integrated on the semiconductor chip, to form a monolithic power IC that is small and light. The monolithic structure minimizes electric characteristic differences between the first and second semiconductor elements.
Each of the first and second semiconductor elements may be formed of FETs that have identical characteristics and are connected in parallel with one another. The number of FETs that form the first semiconductor element may be larger than that of FETs that form the second semiconductor element.
Forming each of the first and second semiconductor elements with FETs that have identical characteristics and are connected in parallel with one another will be explained Each FET has an ON resistance of Rfet. The number of FETs of the first semiconductor element is N1, and that of the second semiconductor element is N2. The ON resistance values RonA and RonB of the first and second semiconductor elements are as follows:
RonA=Rfet/N1xe2x80x83xe2x80x83(5)
RonB=Rfet/N2xe2x80x83xe2x80x83(6)
According to the expressions (4), (5), and (6), the following is obtained:
IDB=(N2/N1)IDAS. . . (7)
In this way, IDB and IDAS are related to each other only with the ratio of the numbers of FETs and become irrelevant to Rfet. Namely, IDB and IDAS are free from a temperature drift in Rfet or variations in manufacturing lots. This results in improving the correctness of break determination. Since N1 greater than N2, IDB less than IDA
As a result, the switching device of the present invention is capable of detecting a current in a power line without a shunt resistor connected in series to the power line, thereby minimizing a heat loss. In addition, the switching device of the present invention needs no microcomputer, thereby reducing the space and cost thereof.
The first semiconductor element may be a power element having a multi-channel structure consisting of parallel-connected unit cells (transistor cells). The numbers of unit cells of the first and second semiconductor elements are adjusted so that the current capacity of the first semiconductor element will be greater than that of the second semiconductor element. The numbers of unit cells determine a current dividing ratio of the first and second semiconductor elements. For example, the second semiconductor element may be made of one unit cell, and the first semiconductor element of 1000 unit cells. In this case, the ratio of the channel width of the second semiconductor element to that of the first semiconductor element is 1:1000, which determines a current dividing ratio. This arrangement enables the second semiconductor element to be small.
Other and further objects and features of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described in connection with the accompanying drawings or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing of the invention in practice.